This made me thing a bit! Not good at this time of day (night!).
Using the same database for multiple forums/domains is a possible. Change tables for those bits that are different and share all the rest.
The advantage for users is that their registration is carried over to all forums and they just log in as normal.
So why, I thought, would people want a different username per forum if the point of sharing the db content is to have similar topics/posts over all domains/forums? Sort of defeats the initial point of the shared database, no?
Or I'm getting old..
Des. . .